In known mixing machines, the arms of the mixing mechanism, which may for example, be spirally distributed on the shaft, are so arranged that the mixing tools, which may, for example be of ploughshare form, can brush over the entire internal wall of the cylindrical container without being obstructed on the opposite side. The mixing tools passing successively, so to speak, in a specific sequence through the material to be mixed which is located in the mixing container thoroughly mix the material and also convey it in a predetermined direction in the mixing container, for example to the outlet located at one end. Depending upon the adjustment of the mixing tools or the arrangement of their lateral flanks, the conveying action of the mixing tools is greater or lesser and the residence time of the material being mixed in the mixing container is correspondingly shorter or longer. If the material is worked in batches, the residence time of the material being mixed in the mixing container can be adjusted more accurately because the mixing container is simply kept closed until the desired residence time has expired. However, if a continuous process is used in which material being mixed is continuously introduced into the mixing container at one end and delivered from the mixing container at the other end, the residence time cannot be so easily controlled. Instead, it is frequently difficult to attain a sufficiently long residence time of the material being mixed using only the agency of the mixing mechanism, for example by design and/or adjustment of the mixing tools.
In order to increase the residence time of the material being mixed in the mixing container of continuously operating mixing machine, it is known incorporate annular discs acting as shutters in the mixing container or to suspend partitions extending partly across the cross-section of the container. These discs or shutters counteract conveying of the material being mixed to the outlet end if it is too rapid. The suspended partitions can also be used for guiding heating or cooling media if they are made hollow and are connected to a system in which a heating or cooling medium circulates. These known means of increasing the residence time of the material being mixed in continuously operating mixing machines and for heating or cooling the material being mixed are relatively complicated and have the additional disadvantage of complicating the installation and dismantling of the mixing mechanism. The position of these additional fittings must also be determined and maintained very precisely so that it is not possible for them to collide with the mixing tools revolving between them.
It is also known to fix heatable or coolable discs on the shaft of the mixing tool between the mixing tools. It is possible to achieve a heat exchange surface which is as large as possible in the smallest space with an arrangement of this type, but there is the disadvantage of the difficulty in moving the material being mixed using the mixing tools located in the region of the periphery of the discs, which results in poorer mixing and degradation. Furthermore, a mixing mechanism of this type cannot be used for heterogeneous mixing operations with solid-liquid or solid-gaseous phases, and, the axial conveying of the material being mixed by means of the mixing tools is also impaired very considerably.